The present invention relates to gas sorbing composite systems, as well as to methods for their manufacture.
Gas sorbing systems and materials are widely used in industry in all applications where it is necessary to maintain a vacuum or to control the composition of the gaseous atmosphere by the removal of traces of undesired gases, particularly in displays being used as screens for television, computers, or in many other electronic applications, such as hand held computers, mobile phones and the like. A particularly important case of displays is the OLEDs (Organic Light Emitting Diodes), being described for instance in U.S. Pat. Nos. 5,804,917 and 5,882,761, and in particular those belonging to the latest generation, known as “Top Emission OLED” (TOLED). In the latter, it is foreseen that the light leaves the device passing through the surface which is opposed to the one where the system responsible for forming the image is located, that is from the surface most suitable for housing the getter system. In this case, the getter system obviously must be transparent. Because of their importance, particular reference will be made in the following to these latter type of displays, but the teachings of the invention have a more general applicability, for instance in the case of plasma screens.
Gas sorbing materials used in industry are normally inorganic compounds in a finely dispersed form, in order to increase the specific surface (surface area per weight unit) and thus the capability and speed of interaction with the surrounding gaseous environment. Examples of these materials are alkaline-earth metal oxides, such as CaO and BaO (for moisture sorption), zeolites (for the sorption of different gases, such as moisture, carbon oxides or others, depending on the specific zeolite being used), aluminas or the like. A common problem of these materials is that powders are not provided with sufficient cohesion, such as to form compact bodies. This is particularly true in the case of desiccants after moisture sorption.
The problem is normally addressed by dispersing the sorbing material within a dispersant matrix, which is able to keep material particles in a fixed position, while at the same time allowing gases to pass toward the getter itself. Examples of this solution are reported in numerous documents. Japanese patent application publication JP 61-291021 discloses desiccant materials dispersed in a silicone matrix; U.S. Pat. No. 3,704,806 discloses desiccant compositions comprising zeolites dispersed within a matrix consisting of a thermosetting polymer, such as epoxy resins; U.S. Pat. No. 4,081,397 discloses a desiccant system comprising alkaline-earth metal oxide particles dispersed in an elastomeric polymer; U.S. Pat. No. 5,304,419 discloses desiccant compositions comprising a desiccant material dispersed in a matrix, which can consist of silicone, polyurethanes or similar polymers; U.S. Pat. No. 5,591,379 discloses desiccant compositions comprising a desiccant selected among zeolites, alumina, silica gel, alkaline earth metal oxides, and alkaline metal carbonates, the desiccant being dispersed in a matrix of porous glass or ceramic; U.S. Pat. No. 6,226,890 B1 discloses desiccant systems, wherein a desiccant material (e.g., an alkaline-earth metal oxide) is dispersed in a polymer which in the patent is said to have the property of not decreasing or even increasing water sorption speed by the desiccant material, which may be, for instance, silicones, epoxies, polyamides, polymethacrylates or others; and finally, U.S. Pat. No. 6,819,042 B2 discloses desiccant systems consisting of a desiccant material being dispersed in a resin, for instance selected from polyethylene, polypropylene, polybutadiene and polyisoprene.
A first generic drawback occurring with the known sorbing systems resides in the manufacturing thereof. When moisture sorbers, such as alkaline-earth metal oxides, are employed, particles which are moist due to exposure to air tend to adhere to each other. This degrades their characteristics, and therefore they do no longer have the granulometry and the homogeneous distribution, which are desired in the final sorbing system.
Further, these systems are generally formed by suspending the particles of the sorbing material in the material which will form the matrix when it is still liquid (for example, an organic material before polymerization or a molten polymer). The different densities and surface energies of particles and matrix material may cause de-mixings in the suspension, unless the system is kept under continuous stirring, at least from the time of the formation of the suspension until the matrix reaches a viscosity sufficient to prevent the sedimentation of the solid particles. However, this represents an evident complication of the manufacturing processes for these systems.
Further, systems comprising a matrix, obtained by the above described processes, will have sorbing particles that will show a tendency to aggregate during the matrix consolidation.
In addition, when a certain sorbing system is given and it is desired to modify the sorption characteristics thereof, in particular sorbable gases, it is necessary to provide a new preparation by changing the nature of the sorbing particle. This generally involves the need to undertake from the beginning the study of the Theological properties of the system and of how these evolve during the manufacturing thereof, in order to ensure the achievement of a homogeneous and stable particle dispersion.
Finally, as a consequence of gas sorption, these systems may change their overall physical properties, particularly optical ones, such as the refraction index or the light radiation absorption, due to chemical-physical variations of the sorbing particles. This last point is particularly critical. In fact, the getter systems of the prior art do not have optical properties that guarantee their satisfactory use in the previously mentioned TOLED screens.